Well, it finally happened to me. A good 'ol FTE.
I told you so!
It appears to have been due to the case itself. It was the first round of the second box of 20 that day, the chamber was spotless as far as I could tell and the extractor tore off part of the rim of the case. Of course I forgot to bring a cleaning rod with me (like I had intended to "just in case") so my shooting was over. The case tapped out easily.
I fed the other 19 left from that box through manually without incident after I got home. I re-fed the spent case, and it got stuck again. I'll get the calipers here in a bit to measure and see what the culprit is.
It's not something that'll make me stop using the ammo altogether, but it reinforces the idea that this kind of ammo is for plinking only.